Various different processes or techniques are known in the art for producing carbon black. One such process, sometimes referred to as a furnace carbon black producing process, employs a furnace having a burner or combustion chamber followed by a reactor. A combustion gas feed stream, typically a hydrocarbon gas stream such as natural gas, or the like, is combusted in the burner portion of the furnace along with an oxidant feed gas stream such as air or oxygen, to produce hot combustion gases which pass then to the reactor portion of the furnace. In the reactor, hydrocarbon feedstock is exposed to the hot combustion gases. Part of the feedstock is burned, while the rest is decomposed to carbon black. The reaction products typically are quenched to a temperature of about 230° C., whereupon the carbon black content is collected by any of various conventional methods. It is widely recognized that the furnace carbon black process does not operate at desirable efficiency levels. Employing air and natural gas as the feedstreams to the bumer, efficiency typically would not exceed sixty percent (60%) for a lean burn operation, that is, an operation in which the natural gas is feed at less than stoichiometric amount relative to the oxygen content of the air feed stream.
It is desirable to have the hot combustion gases produced in the burner at temperatures sufficiently high to effect efficient pyrolysis of the hydrocarbon feedstock, also referred to as the “make hydrocarbon” to produce carbon black, while not having the temperature of the combustion gases excessively high, which could cause damage to the refractory lining of the combustion zone and/or the reaction zone. Operating at nearly stoichiometric conditions, with feed rates high enough to meet throughput requirements and other necessary operating conditions may produce excessively high temperatures. Operating under fuel rich conditions, that is, employing an excess of the natural gas relative to the air or other oxidant gas stream, may produce tolerable combustion gas temperatures coupled with improved yield, even yield in excess of sixty percent (60%). Under such operating conditions, however, it has been found that the raw material costs generally are uneconomically high. Accordingly, it is typical to operate under a fuel lean combustion strategy to achieve adequate throughput and tolerable raw material costs, notwithstanding the resultant low carbon black yield.
The off-gas or tailgas produced along with the carbon black, due to the nature of the furnace carbon black producing process, has long be recognized to have significant energy value. In a furnace carbon black producing process, tailgas from the reactor filter system, that is, tailgas from which the carbon black has been removed, typically contains combustible gas components. It has long been recognized to be highly desirable to exploit the energy content of such off-gas. It has been used, for example, to preheat the combustion gas feed streams to the burner in a furnace carbon black producing process. It has also been used to preheat the hydrocarbon feedstock fed to the reactor. In addition, the tailgas from a furnace carbon black producing process has been burned to operate a generator to produce electricity for use at the carbon black production facility or for export.
It has also been suggested to use such tailgas as all or part of the combustion gas feed stream fed with the oxidant gas feed stream to the burner in a furnace carbon black producing process that is, it has been suggested to exploit the energy connect of the off-gas by combusting it with an oxidant gas feed stream in the combustion chamber of a carbon black furnace to produce combustion gases for the pryolysis of the hydrocarbon feedstock. Numerous problems are encountered, however, in attempting to produce carbon black by the combustion of such furnace off-gas, including problems in controlling the temperature of the combustion gases, oxygen content of the combustion gases and flow rate of the combustion gases. Such problems have resulted in little if any commercial implementation of furnace carbon black producing processes employing furnace off-gases as a combustion gas feed stream to the same or a different carbon black producing furnace. Exemplary of such prior attempts are U.S. Pat. No. 2,796,332 to Pollock, wherein off-gas is recirculated from a carbon black furnace to its own combustion chamber. The tailgas is preheated and treated for the removal of carbon dioxide. The removal of carbon dioxide adds cost and complexity to the production process. In U.S. Pat. No. 4,261,964 to Scott TV et al, hydrogen gas and carbon monoxide gas are stripped from carbon black furnace off-gas and used to replace natural gas fed to the combustion chamber of the same furnace. In U.S. Pat. No. 3,645,685 to Crouch, carbon black furnace off-gas is recirculated to the reactor portion of the same furnace. In U.S. Pat. No. 4,315,894 to Austin, off-gas from the same or other carbon black furnace is used as a quench fluid. Typically the off-gas in such prior teachings is employed in a fuel lean combustion strategy, seemingly following the fuel lean strategy typically employed when natural gas or other high BTU content combustion gas is employed. U.S. Pat. No. 4,393,034 to Smith is exemplary of this, employing dewatered off-gas in a fuel lean combustion strategy.
It is an object of the present invention to overcome some of the problems encountered in past attempts to employ off-gas from a furnace carbon black producing process as a combustion gas feed stream to the combustion zone of a carbon black furnace. It is a particular object of the invention to provide a commercially feasible use of off-gas from a furnace carbon black producing process as a combustion gas to the burner of the same or a different carbon black furnace operating a furnace carbon black producing process.